Not positive, but "maybe" the yellowing is not from the Copper Fungicide unless you mixed it at a higher rate than the recommended amount or applied more often than recommended? What type of potting mix did you use...another possibility is your mix is staying too wet from all the rainfall you've had... I have mine in very fast draining mix(Al's Gritty Mix) which has helped. I usually have to water mine once a day or every other day. I've been spraying mine with a fungicide according to directions and have had no problems with yellowing leaves. I did have issues almost 3 yrs ago shortly after I repotted mine to the gritty mix, but I bare rooted the tree...removed 100% of the old peat soil. It took about 3 months to fully recover, I lost a lot of leaves some turned yellow, fell off and some leaves fell off that was completely green. I assumed that it was going through shock and was trying to re-establish in the new soil? When I started to see it was in recovery mode and started to see new growths, I added some "weak" fertilizer with a little fish fertilizer and a tsp of Epsom salts to a gallon of water every couple of weeks. Several weeks later, the tree greened up and recovered. Not saying that this is what's happening to your tree, just a "possibility"?

As far as what to do right now...not positive but you are probably going to have to let it do its thing, I'm not sure there is anything you can do to stop the leaves from dropping at this point. If it were mine, I would check the roots(very carefully) to see if the soil is very wet/soggy from all that rain. I would bring it inside in the garage or covered patio, if you know that you will get rain...and let the soil dry out before you water again. Just my humble opinion.

Good luck